Method and system for interworking of cellular networks and wireless local area networks

ABSTRACT

A method and system for interworking between cellular networks and wireless local area networks (WLANs) are disclosed. At least one cellular network, at least one WLAN and an IP network are deployed. A wireless transmit/receive unit (WTRU) first establishes a connection to a WLAN and a tunnel between an access point (AP) and a packet data gateway (PDG) is established. The PDG further establishes a tunnel to an IP network. The WTRU then invokes a service which is delivered through the WLAN. As signal quality from the AP degrades below a predetermined threshold, a handover from the WLAN to the cellular network is performed. A new connection to the cellular network may be established either before or after breaking the current connection to the WLAN or the two connections may be maintained simultaneously.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 60/634,679 filed Dec. 9, 2004, which is incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention is related to wireless communication systems. More specifically, the present invention is a method and system for interworking between cellular networks and wireless local area networks (WLANs).

BACKGROUND

Different types of wireless communication networks are presently deployed, such as WLANs and cellular networks. A multi-mode wireless transmit/receive unit (WTRU) supports wireless communication in more than one wireless communication network. As a user of the multi-mode WTRU roams between different networks, it is necessary to perform handover from one network to the other while receiving services continuously. For example, a wireless subscriber may roam between a WLAN and a third generation (3G) network while maintaining continuity in the wireless service provided to the user. Therefore, there is a need for coordination between the WTRU and the networks such that the service continuity is maintained as the user roams between different wireless networks.

SUMMARY

The present invention is related to a method and system for interworking between cellular networks and WLANs. At least one cellular network, at least one WLAN and an IP network are deployed. The WLAN includes an access point (AP). The cellular network includes a radio access network and a core network. The radio access network includes a Node-B and a radio network controller, and the core network includes a packet data gateway (PDG), a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN).

A WTRU first establishes a connection to a WLAN and a tunnel between an AP and a PDG is established. The PDG further establishes a tunnel to an IP network. The WTRU then invokes a service which is delivered through the WLAN. As signal quality from the AP degrades below a predetermined threshold, a handover from the WLAN to the cellular network is performed. A new connection to the cellular network may be established either before or after breaking the current connection to the WLAN or the two connections may be maintained simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from the following description of preferred embodiments, given by way of example and to be understood in conjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of a UMTS-WLAN architecture;

FIG. 2 is a signaling diagram of a process for access to 3G based services through a WLAN;

FIG. 3 is a signaling diagram of a process for interworking in accordance with a first embodiment of the present invention;

FIG. 4 is a signaling diagram of an alternative process for interworking in accordance with an alternative to the first embodiment of the present invention;

FIG. 5 is a signaling diagram of a process for interworking in accordance with a second embodiment of the present invention;

FIG. 6 is a signaling diagram of an alternative process for interworking in accordance with an alternative to the second embodiment of the present invention; and

FIG. 7 is a signaling diagram of a process for interworking in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout.

When referred to hereinafter, the terminology “WTRU” includes but is not limited to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereinafter, the terminology “Node-B” and “AP” includes but is not limited to a base station, a site controller or any other type of interfacing device in a wireless environment.

The present invention provides methods for maintaining service continuity and seamless handover between a WLAN and a cellular network by defining steps for establishing the cellular network connectivity, steps for performing a handover, and steps for breaking the connectivity between the user and the WLAN. It should be noted that the cellular network can be any type of cellular network including, but not limited to, a universal mobile telecommunication system (UMTS), cdma2000 and a global system for mobile communication (GSM), and the WLAN can be any type of WLAN including, but not limited to, an IEEE 802.x network.

FIG. 1 is a block diagram of a UMTS-WLAN interworking network 100. WLANs 130 a, 130 b, (e.g., WLAN hot spots), are deployed in the coverage area of the UMTS 110. Each WLAN 130 a, 130 b includes at least one AP 132 a, 132 b for radio access. The AP 132 a, 132 b is connected to an access router (AR) 134 for access to external networks, such as an IP network 140, (e.g., Internet), or a cellular core network 120 for 3G-based services through the WLAN hotspot.

Base stations 112 are deployed in the UMTS coverage area for access to UMTS networks. The base station 112 is connected to a radio network controller (RNC) 114 which is connected to the cellular core network 120.

The cellular core network 120 comprises a circuit switched core network (not shown) and a packet switched core network (shown in FIG. 1). The packet switched core network 120 comprises an SGSN 122, an authentication, authorization and accounting (AAA) server 124, a home location register (HLR)/home subscriber server (HSS) 126, a GGSN 128, a PDG 129 and a WLAN access gateway (WAG) 121.

Referring to FIGS. 1 and 2, a process 200 for access to 3G-based services through WLAN is explained hereinafter. A WTRU 102 is currently in a service area of the WLAN hotspot 130 a. The WTRU 102 acquires system information of the WLAN hotspot 130 a through active or passive scanning (step 202). In active scanning, the WTRU 102 sends a probe request to the AP 132 a and the AP 132 a sends a probe response in response to the probe request (steps 202 a, 202 b). The WTRU 102 may receive beacons from more than one AP. In such case the WTRU typically selects the AP having the strongest signal. In passive scanning, the WTRU 102 listens to the beacon transmitted from the AP 132 a periodically (step 202 c).

After acquiring the system information, WLAN association and authentication procedures are performed. The WTRU 102 sends an association request message to the selected AP 132 a (step 204) and the AP 132 a sends an association response message to the WTRU 102 (step 206). At such point, an association is established and WLAN authentication procedure is performed (step 208).

The WTRU 102 then initiates subscription and service authentication procedures by registering with the UMTS network for receiving UMTS-based services through the WLAN 130 a (step 210). The WLAN 130 a resolves the Network Access Id (NAI) provided by the WTRU 102. The AR 134 uses the NAI to route AAA messages to the relevant AAA server 124 in the UMTS core network 120. The AR 134 triggers extensible authentication protocol (EAP)-authentication key agreement (AKA) authentication and relay messages to a UMTS AAA server 124. Once the WTRU 102 receives an authentication success message, the WTRU 102 uses dynamic host configuration protocol (DHCP) to receive an IP address and then initiates a tunnel establishment with the PDG 129 through the WAG 121. The WTRU 102 constructs a fully qualified domain name (FQDN) and performs a domain name service (DNS) query for the PDG 129 from a DNS 142 (step 212). The WTRU 102 selects a PDG from the received list in the DNS query response and establishes an end-to-end tunnel between the selected PDG 129 and the WTRU 102 (step 214).

FIG. 3 is a signaling diagram of a process 300 for interworking in accordance with the first embodiment of the present invention. In accordance with the first embodiment, a new connection to the UMTS network is established before breaking the current connection to the WLAN hotspot, (i.e., “make before break”). When establishing the tunnel at step 214 in FIG. 2, the WTRU indicates an application, such as voice over IP (VoIP) services, and the tunnel is set up for this certain application. The tunnel is established by the WTRU 102 sending a request to the AP 132 a (step 302 a) and the AP 132 a forwarding the request to the PDG 129 (step 302 b). After the tunnel between the WTRU 102 and the PDG 129 is established, the WTRU 102 invokes the indicated service (step 302).

There are two options that may follow the indication of the application. One is that a request is sent to the PDG 129 to establish the connection to the IP Multimedia Subsystem (IMS) 150 and allocate the Proxy Call State Control Function (P-CSCF) or the Session Initiation Protocol (SIP) proxy for the WTRU 102. The other option is that a request is sent to the PDG 129 to establish the tunnel and wait for the WTRU 102 to request a connection to the IMS 150 and the allocation of the SIP proxy or the P-CSCF is performed after the request for connection. The first option is preferred since it will save additional delay in setting up the call. However, the second option may be the implementation in certain situations. The step 304 between the PDG 129 and the IMS 150 indicates the steps taking place to establish the connection between the PDG 129 and the IMS 150, such as SIP registration, allocation of P-CSCF and the allocation of Serving CSCF (S-CSCF). A CSCF is a specific type of SIP server, which is used to process SIP signaling packets in an IMS network. A P-CSCF is an SIP proxy that is the first point of contact for the WTRU. An S-CSCF is a central node of the signaling plane.

As the WTRU 102 moves away from the current WLAN hotspot 130 a, as shown in FIG. 1, a handover from the current WLAN hotspot 130 a to the UMTS network 110 is initiated. In accordance with this embodiment, a new connectivity to the UMTS network 110 is established before breaking the existing connectivity to the current WLAN hotspot 130 a.

Referring again to FIG. 3, the WTRU 102 establishes a connection to the GGSN 128 as indicated by arrow 305 by the following steps 306-310. The WTRU 102 first establishes a radio access bearer (RAB) to a Node-B 112 (step 306) and invokes a 3GPP system attachment (step 308). The WTRU 102 then invokes 3GPP IP connectivity by establishing a packet data protocol (PDP) context (step 310). When the WTRU 102 sets up a PDP context, the WTRU 102 selects an access point and an access point name (APN) is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN 128 which serves the access point. The WTRU 102 then invokes 3GPP IMS connectivity through SIP registration at step 312 at such point the connection between the GGSN 128 and the IMS 150 is also established as indicated by arrow 312 a.

Once the connectivity to the UMTS network 110 is established, a process for breaking the connectivity to the current WLAN hotspot 130 a is initiated. The WTRU 102 sends a handover request to the AP 132 a (step 314). The handover request identifies the tunnel end points, the user ID, radio resources, frequency channels, priority, or the like. The AP 132 a then sends a 3GPP relocation request to the PDG 129 (step 316). There are two options with respect to the 3GPP relocation request. The PDG 129 may be removed from the call path after the connectivity to the WLAN 130 a is terminated or the PDG 129 may remain on the call path after the connectivity to the WLAN 130 a is terminated. FIG. 3 illustrates the first option and the second option will be explained with reference to FIG. 4 hereinafter.

In the first embodiment shown in FIG. 3, the PDG 129 is removed from the call path after the connectivity to the WLAN 130 a is terminated. The PDG 129 forwards the request to the GGSN 128, and the GGSN 128 forwards the request to the IMS 150 (steps 318, 320). The tunnel between the PDG 129 and the GGSN 128 lasts only for the duration the connectivity to the WLAN 130 a exists, and then a new connection between the GGSN 128 and the IMS 150 is established and traffic is forwarded directly from the IMS 150 to the GGSN 128 where the WTRU 102 is now connected.

The IMS 150 sends a relocation response to the GGSN 128, which forwards the response to the PDG 129 (steps 322, 324). The PDG 129 sends a relocation response to the AP 132 a (step 326). The AP 132 a then releases the resources after sending a handover complete message to the WTRU 102 (step 328). The GGSN 128 also sends the handover complete message, (i.e., HO complete), for resource allocation to the Node-B 112 via the SGSN 122 (steps 330, 332). The Node-B 112 then sends the handover complete message to the WTRU 102 (step 334). The services from the IMS 150 are then provided through the UMTS network 110, (i.e., from the IMS 150 via the GGSN 128, the SGSN 122 and the Node-B 112 to the WTRU 102 as indicated by arrows 336 a-336 c) (steps 336, 338).

FIG. 4 is a signaling diagram of an alternative process 400 to the first embodiment. Process 400 is similar to process 300 except the PDG 129 remains on the call path after the connectivity to the WLAN 140 a is terminated. The PDG 129 will be in the middle of the call path after the handover. The handover is performed by switching the signaling path in the P-CSCF toward the GGSN 128 from the PDG 129. The traffic is directed from the PDG 129 to the GGSN 128.

Steps 402-416 are the same as corresponding steps 302-316 and will not be repeated herein. After receiving the relocation request from the AP 132 a, the PDG 129 sends a tunnel establishment request to the GGSN 128 and the GGSN 128 responds with a tunnel establishment response. At such point a tunnel is established between the PDG 129 and the GGSN 128. The GGSN 128 establishes the SIP connectivity to the IMS 150 through the PDG 129 (steps 422, 424). The PDG 129 sends a relocation response to the AP 132 a (step 426). The AP 132 a then releases the resources after sending a handover complete message to the WTRU 102 (step 428). The GGSN 128 also sends the handover complete message for resource allocation to the Node-B 112 via the SGSN 122 (steps 430, 432). The Node-B 112 then sends the handover complete message to the WTRU 102 (step 434). The services from the IMS 150 are then provided through the UMTS network 110, (i.e., from the IMS 150 via the PDG 129, the GGSN 128, the SGSN 122 and the Node-B 112 to the WTRU 102 as indicated by arrows 436 a-436 c) (step 436).

FIG. 5 is a signaling diagram of a process 500 for interworking in accordance with a second embodiment of the present invention. In accordance with the second embodiment, the WTRU 102 may maintain multiple sessions simultaneously and the existing connectivity to the WLAN 130 a is not torn down after the handover is complete. Two connections are maintained simultaneously and the application is transferred from one network to the other, (i.e., “simultaneous”).

After the tunnel between the WTRU 102 and the PDG 129 is established, the WTRU 102 invokes a service, such as VoIP call services (step 502). The WTRU 102 sends a request to the AP 132 a (step 502 a) and the AP 132 a forwards the request to the PDG 129 (step 502 b). The step 504 between the PDG 129 and the IMS 150 indicates the steps taken place to establish the connection between the PDG 129 and the IMS 150, such as SIP registration, allocation of P-CSCF and the allocation of S-CSCF.

The WTRU 102 establishes an additional connection to the UMTS network 110 concurrently. The WTRU 102 establishes a connection to the GGSN 128 as indicated by arrow 505 by the following steps 506-510. The WTRU 102 establishes an RAB to a Node-B 112 (step 506) and invokes a 3GPP system attachment (step 508). The WTRU 102 then invokes 3GPP IP connectivity by establishing a PDP context (step 510). When the WTRU 102 sets up a PDP context, the WTRU 102 selects an access point and an APN is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN 128 which serves the access point. The WTRU 102 then invokes 3GPP IMS connectivity through SIP registration at step 512 at such point the connection between the GGSN 128 and the IMS 150 is also established as indicated by arrow 512 a.

As the WTRU 102 moves away from the current WLAN hotspot 130 a, as shown in FIG. 1, the application is transferred from the WLAN 130 a to the UMTS network 110 without breaking the existing connection to the WLAN 130 a. The WTRU 102 sends a handover request to the AP 132 a (step 514). The handover request identifies the tunnel end points, the user ID, radio resources, frequency channels, priority, or the like. The AP 132 a then sends a 3GPP relocation request to the PDG 129 (step 516). As stated hereinbefore with respect to the first embodiment and its alternative, the PDG 129 may be removed from the call path after the connection is switched to the UMTS or may remain on the call path. FIG. 5 illustrates the first option and the second option will be explained with reference to FIG. 6 hereinafter.

The PDG 129 forwards the request to the GGSN 128, and the GGSN 128 forwards the request to the IMS 150 (steps 518, 520). The PDG 129 is removed from the call path after the connectivity to the WLAN 130 a is switched. The tunnel between the PDG 129 and the GGSN 128 lasts only for a certain interval, and a new connection between the GGSN 128 and the IMS 150 is established and traffic is forwarded directly from the IMS 150 to the GGSN 128 where the WTRU 102 is connected.

The IMS 150 sends a relocation response to the GGSN 128, which forwards the response to the PDG 129 (steps 522, 524). The PDG 129 sends a relocation response to the AP 132 a (step 526). The AP 132 a then releases the resources after sending a handover complete message to the WTRU 102 (step 528). The GGSN 128 also sends the handover complete message for resource allocation to the Node-B 112 via the SGSN 122 (steps 530, 532). The Node-B 112 then sends the handover complete message to the WTRU 102 (step 534). The services from the IMS 150 are then provided through the UMTS network 110, (i.e., from the IMS 150 via the GGSN 128, the SGSN 122 and the Node-B 112 to the WTRU 102 as indicated by arrows 536 a-536 c) (steps 536, 538).

FIG. 6 is a signaling diagram of a process 600 which is an alternative to the second embodiment of the present invention. Process 600 is similar to process 500 except the PDG 129 remains on the call path after the connectivity to the WLAN 130 a is switched. The PDG 129 will be in the middle of the call path after the handover.

Steps 602-616 are the same as corresponding steps 502-516 of process 500 and will not be repeated herein. After receiving the relocation request from the AP 132 a, the PDG 129 sends a tunnel establishment request to the GGSN 128 and the GGSN 128 responds with a tunnel establishment response (steps 618, 620). At such point a tunnel is established between the PDG 129 and the GGSN 128. The GGSN 128 establishes the SIP connectivity to the IMS 150 through the PDG 129 (steps 622, 624). The PDG 129 sends a relocation response to the AP 132 a (step 626). The AP 132 a then releases the resources at step 629 after sending a handover complete message to the WTRU 102 (step 628). The GGSN 128 also sends the handover complete message for resource allocation to the Node-B 112 via the SGSN 122 (steps 630, 632). The Node-B 112 then sends the handover complete message to the WTRU 102 (step 634). The services from the IMS 150 are then provided through the UMTS network, (i.e., from the IMS 150 via the PDG 129, the GGSN 128, the SGSN 122 and the Node-B 112 to the WTRU 102 as indicated by arrows 636 a-636 c) (step 636).

FIG. 7 is a signaling diagram of a process 700 for interworking in accordance with a third embodiment of the present invention. In accordance with the third embodiment, the existing connectivity to the WLAN 130 a is torn down before handover to the UMTS network 110, (i.e., “break before make”). After the tunnel between the WTRU 102 and the PDG 129 is established, the WTRU 102 invokes the indicated service (step 702). To invoke the indicated service, the WTRU 102 sends a request to the AP 132 a (step 702 a) and the AP 132 a forwards the request to the PDG 129 (step 702 b). The step 704 between the PDG 129 and the IMS 150 indicates the steps taken place to establish the connection between the PDG and the IMS, such as SIP registration, allocation of P-CSCF and the allocation of S-CSCF.

As the WTRU 102 moves away from the current WLAN hotspot 130 a, as shown in FIG. 1, handover from the current WLAN hotspot 130 a to the UMTS network 110 is performed. In accordance with this embodiment, a new connectivity to the UMTS network 110 is established after breaking the existing connectivity to the current WLAN hotspot 130 a, (e.g., loss of signal).

When the signal from the AP 132 a is lost (step 706), the WTRU may initiate the handover to the UMTS system or alternatively the WLAN may initiate the handover. Since the WLAN is connected to the PDG 129, the WLAN may initiate the handover to the target UMTS system. When the signal loss is detected, the AP 132 a sends a message, (a relocation request), to the PDG 129 (step 708). The session is then maintained for a certain interval (step 710).

The WTRU 102 then establishes a connection to the GGSN 128 as indicated by arrow 711 by the following steps 712-716. The WTRU 102 establishes an RAB to a Node-B 112 (step 712) and invokes a 3GPP system attachment (step 714). The WTRU 102 then invokes 3GPP IP connectivity by establishing a PDP context (step 716). When the WTRU 102 sets up a PDP context, the WTRU 102 selects an access point and an APN is determined. The APN is used in a DNS query. This process finally gives an IP address of the GGSN 128 which serves the access point. The WTRU 102 then invokes 3GPP IMS connectivity through SIP registration at step 718, at such point the connection between the GGSN 128 and the IMS 150 is also established as indicated by arrow 718 a.

A handover bending session is then initiated (step 720). The WTRU 102 sends the information related to the existing session to the IMS 150, (i.e., SIP server). The information includes the session/service identification, originating and terminating IP addresses, a request to redirect the traffic to the UMTS system with the new contact information, (i.e., current IP address), or the like. The IMS 150 then updates the new routing of the call/session. The IMS 150 establishes a new P-CSCF and S-CSCF for the new session.

The IMS 150 then sends a handover request notification to the PDG 129 with information regarding the session and indications that the call/session has been redirected and resources previously reserved should be released (step 722). The PDG 129 then sends a relocation response to the AP 132 a along with the session information and WTRU identity (step 724). The AP 132 a then releases resources allocated for the WTRU 102. The session is resumed between the WTRU 102 and the IMS 150 (steps 726 a-726 d) and user invoked services are provided from the IMS 150 via the GGSN 128, the SGSN 122 and the Node-B 112 to the WTRU 102 (step 728).

The PDG 129 may indicate a handover to the IMS 150. Alternatively, the WTRU 102 may indicate the handover to the IMS 150 and provide the old connection information.

Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. 

1. In a wireless communication system including at least one cellular network, at least one wireless local area network (WLAN) and an IP multimedia subsystem (IMS), a method for interworking between the cellular network and the WLAN for a user service provided by the IMS, the method comprising: a WTRU establishing a connection to a WLAN; establishing a tunnel between the WLAN and a packet data gateway (PDG) of the cellular network, the PDG further establishing a tunnel between the PDG and an IP network; the WTRU invoking a user service and receiving the user service through the WLAN; the WTRU establishing a connection to the cellular network; and performing a handover from the WLAN to the cellular network by sending a handover signaling through the WLAN, whereby the user service is continuously provided through the cellular network after the handover.
 2. The method of claim 1 wherein the PDG sends a relocation request to a gateway GPRS support node (GGSN) when the PDG receives the handover request, whereby the PDG is removed from a call path after the handover is performed, and the call path between the IMS and the WTRU is established through the GGSN.
 3. The method of claim 1 wherein the PDG sends a tunnel establishment request to a gateway GPRS support node (GGSN) when the PDG receives the handover request, whereby the PDG remains on a call path after the handover is performed, and the call path between the IMS and the WTRU is established via the GGSN and the PDG.
 4. The method of claim 1 wherein the tunnel between the PDG and the IMS is established after indication of the service.
 5. The method of claim 1 wherein the tunnel between the PDG and the IMS is established after a request from the WTRU.
 6. The method of claim 1 wherein the handover request identifies tunnel end points.
 7. The method of claim 1 wherein the handover request identifies user identity (ID).
 8. The method of claim 1 wherein the handover request identifies radio resources.
 9. The method of claim 1 wherein the handover request identifies frequency channels.
 10. The method of claim 1 wherein the handover request identifies priority.
 11. The method of claim 1 wherein the connection to the WLAN is terminated after the connection to the cellular network is established.
 12. The method of claim 1 wherein the connection to the WLAN and the connection to the cellular network are maintained simultaneously.
 13. In a wireless communication system including at least one cellular network, at least one wireless local area network (WLAN) and an IP multimedia subsystem (IMS), a method for interworking between the cellular network and the WLAN for a user service provided by the IMS, the method comprising: a WTRU establishing a connection to a WLAN; establishing a tunnel between the WLAN and a packet data gateway (PDG) of the cellular network, the PDG further establishing a tunnel between the PDG and an IP network; the WTRU initiating a session for a user service and receiving the user service through the WLAN; the WTRU establishing a connection to the cellular network when the connection to the WTRU is broken; initiating a handover bending session to recover the session; and performing a handover from the WLAN to the cellular network, whereby the user service is continuously provided through the cellular network.
 14. The method of claim 13 wherein the handover to the cellular network is initiated by the WLAN.
 15. The method of claim 13 wherein the handover to the cellular network is initiated by a user.
 16. In a wireless communication system including at least one cellular network, at least one wireless local area network (WLAN) and an IP multimedia subsystem (IMS), a system for interworking between the cellular network and the WLAN for a user service provided by the IMS, the system comprising: a cellular network comprising a radio access network and a core network, the core network comprising a packet data gateway (PDG), a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN); a WLAN configured to establish a tunnel between the WLAN and the PDG of the cellular network upon initiation of the user service by a user, the PDG further establishing a tunnel between the PDG and the IMS; a dual mode WTRU configured to communicate with both the WLAN and the cellular network, the WTRU comprising a handover function to perform a handover from the WLAN to the cellular network by sending a handover signaling through the WLAN whereby the WTRU continuously receives the user service through the cellular network after handover.
 17. The system of claim 16 wherein the PDG sends a relocation request to the GGSN when the PDG receives the handover request, whereby the PDG is removed from a call path after the handover is performed, and the call path between the IMS and the WTRU is established through the GGSN.
 18. The system of claim 16 wherein the PDG sends a tunnel establishment request to the GGSN when the PDG receives the handover request, whereby the PDG remains on a call path after the handover is performed, and the call path between the IMS and the WTRU is established via the GGSN and the PDG.
 19. The system of claim 16 wherein the tunnel between the PDG and the IMS is established after indication of the service.
 20. The system of claim 16 wherein the tunnel between the PDG and the IMS is established after a request from the WTRU.
 21. The system of claim 16 wherein the handover request identifies tunnel end points.
 22. The system of claim 16 wherein the handover request identifies user identity (ID).
 23. The system of claim 16 wherein the handover request identifies radio resources.
 24. The system of claim 16 wherein the handover request identifies frequency channels.
 25. The system of claim 16 wherein the handover request identifies priority.
 26. The system of claim 16 wherein the connection to the WLAN is terminated after the connection to the cellular network is established.
 27. The system of claim 16 wherein the connection to the WLAN and the connection to the cellular network are maintained simultaneously.
 28. In a wireless communication system including at least one cellular network, at least one wireless local area network (WLAN) and an IP multimedia subsystem (IMS), a system for interworking between the cellular network and the WLAN for a user service provided by the IMS, the system comprising: a cellular network comprising a radio access network and a core network, the core network comprising a packet data gateway (PDG), a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN); a WLAN configured to establish a tunnel between the WLAN and the PDG of the cellular network upon initiation of the user service by a user, the PDG further establishing a tunnel between the PDG and the IMS; a dual mode WTRU configured to communicate with both the WLAN and the cellular network, the WTRU comprising a handover function to perform a handover from the WLAN to the cellular network by establishing a connection to the cellular network when the connection to the WTRU is broken and initiating a handover bending session to recover the session, whereby the WTRU continuously receives the user service through the cellular network after handover.
 29. The system of claim 28 wherein the handover to the cellular network is initiated by the WLAN.
 30. The system of claim 28 wherein the handover to the cellular network is initiated by a user. 